The present invention relates to a tuning circuit, more specifically to a frequency tuning circuit for continuous-time filter capable of making exact Gm value to minimize the variation of the cutoff frequency due to the variation of process in Gm-C type of continuous-time filter.
Generally, because CMOS suffers from severe variation of process as compared to bipolar, in the circuit such as the Gm-C type of continuous-time filter, the drift of the Gm value is induced, causing the cutoff frequency of the filter to drift. Therefore, the Gm tuning in the CMOS Gm-type continuous-time filter is essential.
In the prior art, the circuit is disclosed which performs the tuning by using the filter circuit composed of a oscillator oscillating at the cutoff frequency in tuning mode without using the tuning circuits.
However, although the above-mentioned prior art circuit has the advantages of obtaining an exact tuning value due to the direct use of the filter circuit in tuning without additional tuning circuits, it has problems that trimming resistors or programmable current sources are necessary in order to regulate the bias current, and the filter circuit cannot be employed when the information signal is inputted continuously into the filter circuit with no null state.
Also, the tuning method using the ratio of the amplitudes before and after passing through the filter to the given reference signal is also disclosed in the prior art.
However, the prior art has the problems that only one tuning can be performed under the ON of the power supply of the system if the signal inputted to the system is continuous, and the tuning can be performed only if a signal is in the temporary cutoff, namely null state.
Further, the tuning method is disclosed which a oscillator consists of the same cell as a main filter circuit, and comprises the steps comparing the frequencies and phases of the oscillator and those of a reference signal and making the differences therebetween minimize.
However, the prior art has the problems that cutoff frequency is drifted since the offset generated by the band-pass filter used in the tuning circuit cannot be corrected.
The present invention is for avoiding the above-mentioned problems, and has the object to provide a tuning circuit for continuous-time filter having a target Gm value without being affected by the variation of process by correcting the offset generated in Gm cell of the tuning circuit, and having reduced consumption of electric power and clock feedthrough by intermittently operating the tuning circuit regardless of the operation of the main filter circuit.
In order to accomplish the object, in accordance with the invention, a tuning circuit is provided which comprises:
Integenerating a current multiplied by a voltage difference of the first reference voltage and the second reference voltage, generating a signal discharging from the first reference voltage to a first predetermined value and a signal charging from the second reference voltage to a second predetermined value according to the generated current, and tuning the Gm cells according to a control signal inputted to a control terminal;
Offset sampling means for receiving the current multiplied by the offset voltages from the Gm cells of the integrating means, sampling the offset voltages of the Gm cells by providing a feedback path between output nodes and input nodes of the Gm cells and generating a current multiplied by the sampled offset voltage, and tuning the Gm cells according to the control signal inputted to the control terminal;
Comparative signal generating means for generating a reference signal by dividing a clock inputted from the external, receiving the signal discharging from the first reference voltage to the first predetermined value and the signal charging from the second reference voltage to the second predetermined value from the integrating means, and generating a comparative signal to compare an actual intersection and a target intersection of these signals; and
Control means for generating a control signal to regulate Gm values of the integrating means and the offset sampling means by receiving the reference signal and the comparative signal from the comparative signal generating means and detecting phase differences therebetween